A lithographic photomask, or reticle, is conventionally used for patterning wafers. Reticle defects at critical points in the design may lead to a total chip yield loss. This issue is particularly severe for EUV technology where defect-free reticles are difficult to obtain, as will be explained below. An EUV reticle is formed from an EUV blank, which includes a reflective multilayer dielectric stack (for example, fifty alternating layers) on a non-transmissive (to EUV radiation) substrate. Due to the complex build, it is currently impossible to manufacture EUV blanks that are entirely free of defects. Blank induced EUV reticle defects are often irreparable, but may be avoided by shifting the reticle pattern with respect to the blank to move the blank defects into a non-critical design region. However, this method only works if the design and blank defect map are favorably matched, and if the blank defects are detected prior to manufacturing the reticle. In addition, non-blank induced defects in an EUV reticle cannot be corrected in this manner. Finding blanks that are suitable for a given design takes time and increases cost for the reticles.
Some EUV reticle defects (but only a minority of blank-induced defects) may be repaired. For conventional transmissive reticles, defect repair is often possible, but if the defects are only detected after completion of the manufacturing process, the repair requires in most cases the removal of the pellicle (a thin organic membrane on a metal frame that is glued to the front surface, or active side, of the reticle), which takes time and may damage the reticle. Alternatively, all reticle defects may be corrected by writing a replacement reticle. However, writing a new reticle is expensive and adds significant time to the setup time for a product, as no wafers may be printed while the new reticle is being produced. Further, a new reticle must be qualified again (i.e., checked again for errors), thereby adding more time to the setup time, and may include as many or more defects than the original reticle. For EUV, the new reticle will suffer from the same issue of inherent blank defectivity as the original reticle. The reliable supply of defect-free EUV reticles is at present an unsolved problem for the industry.
Weak spots may also be present in the design or be caused by optical proximity correction (OPC) and may have the same effect as reticle defects. Such detected weak spots may in some cases be corrected in a reticle repair process, but more often require a design re-spin and the writing of a replacement reticle, both of which are costly and time consuming.
In other cases, the need for design modifications, e.g. the addition of contacts, may arise after manufacturing of the reticle. As in the case of design weak spots, such modifications may occasionally be realized by reticle repair processes, but usually require the writing of a replacement reticle. The absence of the required design modifications in the manufactured reticle will be referred to as “design shortcomings” in the following.
A need therefore exists for methodology enabling reticle defect, design weak spot or OPC weak spot correction, or design modifications, to obtain a defect-free pattern with the correct design on a wafer, without the need for a replacement reticle.